Capacitor type translating system having a varying reactance



y 1951 J. o. MESA 2,559,454

CAPACITOR TYPE TRANSLATING SYSTEM I HAVING A VARYING REACTANCE Filed July 18, 1945 3 Sheets-Sheet 1 h m E a 3 M 74 v .D/STHNCEBETWEENPLATES y 3, 1951 J. o. MESA 2,559,454

CAPACITOR TYPE TRANSLATING SYSTEM HAVING A VARYING REACTANCE 3 Sheets-Sheet 2 Filed July 18, 1945 HT mi Patented July 3, 1951 CAPACITOR TYPE TRAN SLATING SYSTEM HAVING A VARYING REACTANCE Joseph 0. Mesa, Bayside, N. Y., assignor to Stewart-Warner Corporation, Chicago, 111., a corporation of Virginia Application July 18, 1945, Serial No. 605,652

6 Claims.

The present invention relates to electrical translating systems and devices, and more particularly to pickup systems and pickups which may be used, for instance, as phonograph pickups.

The primary object of the present invention is the provision of a new and improved translating system and device.

A further object of the invention is to provide a new and improved pickup system and pickup of the capacitance type which is free of distortion.

A further object of the present invention is to provide a new and improved capacitance translating system employing the :change in the reactance of a capacitor as the modulating factor, rather than the change in capacitance.

Another object of the present invention is to provide a new and improved translating system employing a capacitance type pickup having an increased effective depth of modulation.

A more specific object of the present invention is the provision of a new and improved translatin system including a capacitance type pickup supplied with a carrier frequency wherein a portion of the carrier is canceled out to increase the efiective depth of modulation.

Another more specific object of the present invention is the provision of a new and improved translating system including a capacitance type pickup having a capacitor with a central movable plate whereby the modulation components are added and the carrier components are partly canceled further to increase the effective depth of modulation.

Another object of the invention is the provision of new and improvedcapacitance type pickups characterized by a central movable capacitor plate. 1

Other objects and advantages of the present invention will become more apparent from the ensuing description thereof, in the course of which reference is had to the accompanying drawings, in which Fig. 1 is a graph depicting the change in ca pacitance of a condenser resulting from displacement of the movable plate of the capacitor;

Fig. 2 is a schematic representation of one embodiment of the present invention;

Fig. 3 is a similar representation of a modified embodiment;

Fig. 4 is a similar representation of another (c1. 179 1oo.4)

2 Fig. 7 is a front elevational view of a capaci tance type pickup constructed in accordance with the present invention and especially suited for use in a system of the character illustrated in Fig. 4;

Fig. 8 is a bottom plan view of the pickup; Fig. 9 is a side elevational view of the pickup; Fig. 10 is a vertical cross sectional view of another pickup of a type utilizable in the system of Fig. 4; and

Fig. 11 is a top plan view of the pickup.

Reproducing or translatin systems using capacitance type pickups are known. In one type the pickup delivers a modulation voltage proportional to capacitance. This type of pickup is a source of distortion which may be objectionable under certain conditions. The distortion results from the fact that the capacitance is inversely related to the thickness of the dielectric. This is illustrated in Fig. 1, in which curve l0 depicts the relation between capacitance and the distance between plates or the thickness of the dielectric.

If the displacement of the movable capacitor plate follows a sinusoid, as depicted in curve l2, the capacitance varies in the mannerillustrated by curve M. An analysis of curve M, or of the output curve, indicates the presence of a second harmonic of about 26.5% and a third harmonic of about 7.2%.

The distortion in the output might be corrected as by using a suitable non-linear amplifier or by reducing the depth of the modulation. Thus, if the excursion of the movable plate on each side of its center position were one-twenty-fifth of the distance between plates, the second and third harmonics would have values of about 4.2% and 0.2%, respectively. However, it is desirable to have a large ratio of plate excursion to plate sep-' aration because of the fact that the lower the mechanical transfer impedance, the greater the plate displacement.

The present invention avoids the distortion described above even with deep modulation. The invention is based upon the utilization of a change in reactance of a capacitor, rather than a change in capacitance. Since the change in reactance is inversely proportional to change in capacitance, and the latter is inversely proportional to the change in position of the movable plate of the capacitor, it follows that the change is directly proportional to the displacement of the movable plate. Consequently, non-linear distortion is eliminated.

One embodiment of the invention is illustrated 3 The system includes a high frequency oscillator l6 operating, for example, at a frequency of 465 kc. (a frequency in the pass band of the I. F. amplifier of the receiver with which the device is to be used) supplying current to a capacitance type pickup l8 through a resistor 25!. The resistance of resistor 2:29 is high compared to the reactance of the pickup at the oscillator frequency so that the current through the circuit 701 (di l a sin wet) I01 I11 (l-l-m Sin wet) where m=gl==modulation factor .II the oscillator or carrier voltage .is b sin wet, then the voltage across the pickup is in the form 1C2 sin wet (1+m sin (st) i The voltage vacross the pickup is thus an amplitude modulated carrier wave having a carrier Irequen y equ l to the oscillator frequency a d havin a modulation depth which depends upon the displacement or the movable plate.

. Anincreased efiective depth of modulation for a fixed displacement is Obtained with the embodiment of the invention illustrated in Fig. 3. The variable capacitance pickup I8 having the stationary and movable plates 22 and 24 is connected to a fixed capacitor 28 having a capaci tancesomewhat reater than that of the pick-,-

The junction of the two capacitors is connected to the oscillator 16 and to the junction of the resistors 30 and. 32 having resistance values whichare large compared to the reactances of the capacitors at the frequency of the oscillator. The resistors are connected to the capacitors and both the resistors and capacitors are connected across the output terminals 26.

current flow in the circuit 18, 1.6, .36 is constant, because-of the high resistance of the resistor 3.0, and does not depend upon the capacitancepf the-pickup. The voltage across the capacitor [.8 is directly proportional to its re- ,actance; which,- in turn, is directly proportional to the position (displacement) of the movable plate. Hence. the voltage across capacitor is an amplitude modulated voltage Wave having a carrier frequency determined by the oscillator frequency and having its modulation determined by the movement of the needle actuated movable plate- .7

The current flow in the circuit 28, I6, 32 is also constant but 180 out of phase with the current in circuit [8, I6, 30. The voltage across the fixed capacitor .28 is constant and may be considered to be an unmodulated carrier 180 out of phase with the carrier voltage across capacitor. 1' 8.

The output terminal voltage is the sum of the voltages across capacitors l8 and 28. Because the capacitance of capacitor 28 is larger than that of capacitor 18, a part of the carrier is canceled and the effective depth of modulation across the output terminals is greater than that obtaining across the capacitor I8.

The embodiment of the invention illustrated in Fig. 4 includes a capacitance type pickup 34 having a needle actuated movable center plate 36 and a pair of .iixed plates 38 and 40 connected across output terminals 42. The oscillator I6 is connected to the movable plate 36 and a potentiometer including an adjustable connection .44 and a resistorhaving two portions 46 and 48 at opposite :sides of the adjustable connection are also connected across the pickup. The resistors have resistance values which are high compared to the reactances of capacitors 36--38 and 3640 of the pickup.

or reasons similar to those described in connection with Figs. 2 and 3, thevoltages across each "half of the pickup are amplitude modulated waves having a carrier frequency determined by the frequency of oscillator I6 and having'its modulation determined by the displacement of the movable plate 36.

In this embodiment of the invention, both the carrier and modulation components of the voltages across the pickup halves are out of phase. The output voltage appearing across terminals 42, being the sum of the two voltages, is a modulated wave in which, by proper adjustment, the carrier is partly canceled and the modulation components are added. The proper adjustment, which is accomplished by adjustment of the potentiometer, requires that the currents in the two circuits must be made to have magnitudes such that the instantaneous value of the voltage across the output terminals never reaches zero.

A more concise mathematical description of the operation of the embodiment of Fig. 3 follows, with particular reference to Figs. 5 and 6. I

The voltage across one half (the upper, for example) of the pickup may be expressed, in the notation of rotating vectors, as

e =la1 exp [1 (wc't+'9)]+bl exp [j(wt-:-)] in which (as indicated in Figs. 5 and 6) ai=carrier amplitude, side 1 bi modulation amplitude, side 1 wc=carrier angular velocity ws=modulation angular velocity 6=carrier initial phase angle =modulation initial :phase angle The modulation is, by definition J32 a; I r, The voltage across the other half (lower) the picku is ez=a2 exp [y'(wt+0+1r )]+bz exp mama]; a2 exp [7'(w,5t+0)]+'bz exp [j(wst+4 j) in which laz carrier amplitude, side 2 bz modulatiorn amplitude, side .2

The voltage acrossthe output terminals is e1+e2=(a1a2) exp .[7(wct+.0)l+

(bi-il-bz exp [y'(wst+')l The modulation factor is I (Iv-a2 It 'is apparent, therefore, that the modifia-tio'ri factor can be varied or increasedby suitable adjustment of the magnitudes of m and a2, 1. e., the resistances of resistor portions 46 and 48. To avoid over-modulation distortion it is sufiicient that The pickup, which is indicated as a whole by reference character 50, comprises a central ca"- pacitor plate 52 movable in suitable manner as by a record groove following needle 54 attached to the plate by a vertical support or shaft 5B.

sulating material suitably apertured to receive screws IOI for securing the plates and ring in assembled relationship. The screws are insulated from the upper plate 98 by bushings I02.

The movable plate actuating shaft 90 is slid ably and rotatably received in a bushing I04 located in a horizontal portion I06 of the lower plate I00, and a bushing I 08 held centrally of the pickup by a resilient spider I I0 secured above the upper plate 98 by the screws WI and spaced from the upper plate by an annular ring I I2. An an nular ring H4 is located above the spider better to hold it at its peripheral region.

The shaft 92 is resiliently held against rotation by a hair spring H6 secured to the upper end of the shaft and to a support IIB of insulating material secured above the ring II4 by one of the 7 screws ml.

The upper end of the shaft is rounded to seat 7 in a rounded socket 51, to take the upward thrust of the shaft. The movable plate 52 is prefer ably a strip of Phosphor bronze about A" wideadjusted by a nut 66 secured to the threaded end I of a bolt 68 passing through the upper ends of the end plates. The end plates are spaced apart by a top plate I0 having reduced end portions 12 passing through suitable apertures in the end plates, whereby the end plates are angularly movable to adjust the tension applied to plate 52.

The capacitor'includes the stationary plates.

14 and 76 located at opposite sides of the central movable plate 52. They are secured to downwardly extending supports 80 of insulating material secured to the underside of the top plate 10 by screws 82. The plates I4 and 76 are secured to the insulating support by screws 84 and the plates are provided with terminal extensions 85.

The pickup illustrated in Figs. 7, 8, and 9 thus comprises a central movable plate located between two stationary plates. When the needle in cooperative relation with a record groove; the movable plate 52 is displaced laterally according to the modulation of the record in the manner of the pickup 34 illustrated schematically in Fig. 4.

The pickup illustrated in Figs. 10 and 11 is co structed and arranged to have a very low natural period of vibration, e. g., about one cycle per second. It is also intended to be used in a different type of apparatus, such as wheel balancing equipment, wherein the movable part of the pickup capacitor is moved in response to vibrations generated by the vibrations of the wheel.

Referring now to Figs. 10 and 11, it may be noted that the capacitor type pickup, indicated as a whole by reference character 90, includes a cylindrical central movable plate 92 of substantial size and preferably made of lead so as to be quite heavy. It is secured as by a set screw 94 to an intermediate portion of a vertically movable actuating shaft 96 mounted upon and insulatedfrom a pair of stationary capacitor plates 98 and I 00. The's'ta'tionary capacitor plates are separated from each other by an annular ring 99 of in- The movable capacitor plate 92 is held centrally in spaced relation to the capacitor plates 98 and I00 by resilient means including a helical spring I20 supporting the shaft. The lower end of the spring rests upon an externally threaded adjustably mounted supporting nut I22 secured inside of a tubular extension I 24 of the capacitor plate I00. The upper end of the spring supports structure fixedly secured to the shaft 95 and consisting of a circular collar I25 and and an externally flanged spring abutment I28 preferably made of insulating material. The spring I20 is adjusted by rotation of the nut I22 centrally to locate the movable plate 92, e. g., so that it hasv a clearance of about .005 relative to the two stationary plates 98 and I00.

In use, the pickup is secured to the vehicle so that the housing, including the stationary plates 98 and I00, is stationary with respect to the ve hicle. The plate 92 moves relative to the plates 98 and I00 in response to the vibrations induced. by rotation of the vehicle wheel, thereby to pro duce the displacement between the stationary and movable plates contemplated with the system of. Fig. 4.

While the present invention has been described in connection with the specific details of several.

embodiments thereof, it should be understood that these details are not intended to be limitative of the invention except in so far as set forth in the accompanying claims.

Having thus described my invention. what I claim as new and desire to secure by United States Letters Patent is as follows:

1. A capacitance type pick-up, including in combination, a relatively thin and narrow ribbon of conductive material constituting a movable capacitor plate, means including a pair of slotted end plates through which the ribbon extends at its ends and a top plate movably supporting said end plates, and means securing said end plates to said top plate in adjusted position for maintaining said ribbon under tension, a needle carrying shaft secured to said ribbon for moving it, and a pair of stationary capacitor plates located one at each side of said movable ribbon and secured to said top plate.

2. Apparatus for translating vibrations into electrical signals, comprising a source of carrier voltage having a frequency much greater than the highest frequency of the vibrations, a vibration pickup including a capacitor which is variable in response to the vibrations, means including a resistance meansconnecting the pickup to said source for producing a flow of carrier frequency current through the capacitor, and a plurality of output terminals connected with said capacitor to derive electrical signals that develop including a pair of stationary electrodes and an electrode movable in response to the vibrations, the electrodes thereby constituting a pair of variable capacitors, resistance providing means connected across the stationary electrodes, a

source of carrier frequency voltage connected with the resistance providing means and with the .movable electrode to produce carrier frequency currents through the capacitors respectively, the carrier source having a frequency much higher than the highest vibration frequency, and a pair of output terminals connected across the stationary electrodes for deriving output signals, the value of the resistance providing means being much greater than the reactances of the capacitors at the carrier frequencyso that the carrier frequency currents are substantially constant and are "not affected appreciably by vibratory variations of the capacitors.

4. Apparatus for translating vibrations into electrical signals, comprising a vibration pickup including a pair of stationary electrodes and an electrode movable in response to vibrations, the electrodes thereby constituting a pair of series connected variable capacitors, an adjustable resistance potentiometer connected across the stationary electrodes, a source of carrier frequency connected with the movable electrode and with the potentiometer .to produce carrier frequency currents through the capacitors, and a pair of output terminals connected across the stationary electrodes for deriving output signals, the resistance of the potentiometer being much greater than the reactances of the capacitors at the carrier frequency so that the carrier frequency currents are not afiected appreciably by vibratory variations of thecapacitors butsaid currents are substantially constant.

5. A capacitance type pickup, comprising a movable capacitor electrode including arelatively thin and narrow conductive ribbon having ends adapted to be held for tensioning the ribbon, a pair of transverse end plates constituting tensionin levers having slots in their lower ends through which the ends of the ribbon respectively, extend, a top fulcrum :platespacing apart the 'endlplates, a tension member adjustable to tilt the upper ends of the end plates inwardly for maintaining the ribbon under tension, and a shaft secured to the ribbon intermediate its ends and having a'record needle at its lower end, the top plate being provided with a socket for receiving the upper end of the shaft, and 7a.:pair of stationuary capacitor electrodes securedtozthe top plate and positioned respectively adjacent each side of the ribbon.

6. Apparatus for translating vibrations "into e'lectricalsignals, comprising a source of carrier frequency voltage, a pair of capacitors connected in series, one of said capacitors having a fixed capacitance and the other being variable :response to vibrations, a pair of series connectedresistors connected across the series connected capacitors, means connecting the source across'the junction of the resistors and the junction of the capacitors for producing carrier frequency currents through the respective capacitors, and @a pair of output terminals connected across the series connected capacitors :for deriving output signals, the resistances of said resistors respectively being much greater than the reactances of said capacitors respectively so that thecarrier frequency currents through the respective capacitors are substantially constant .regardlessof vibratory variations of the variable capacitor.

JOSEPH O MESA.

REFERENCES CITED The "following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,702,438 High 'Feb. 19, 1929 1,755,739 Kositzky Apr. v22, 1930 1,786,436 Leffler Dec. 30, 1930 1,796,155 Leiiler Mar. 10, 1931 1,799,974 Conrad Apr. 7, 1931 2,008,713 Hayes July 23, 1935 2,055,736 Terman Sept. 29, 1936 2,140,769 Schienemann Dec. 20, 1938 2,182,377 Guanell'a Dec. 5, 1939 2,212,389 Chorpening Aug. 20, 1940 2,252,057 Blessing Aug. 12, 1941 2,319,622 Miessner May 18, 1943 2,376,456 Sinnett May 22, 1945 2,386,049 Hausz Oct. 2, 1945 

